A. Field of Invention
This invention pertains to the art of methods and apparatuses regarding vehicle automatic transmission systems, and more particularly to methods and apparatuses regarding autonomous throttle control during manual downshifting at low accelerator pedal openings.
B. Description of the Related Art
It is known to couple an engine to an automatic transmission via an input shaft and a torque converter. The torque converter is a fluid coupling which allows the engine and the input shaft to turn at different speeds or rotations per minute (RPM). The automatic transmission includes a plurality of gears and associated components that are manipulated to drive an output shaft using different gear ratios such that the torque transferred from the engine is either increased or reduced. The gear ratio is the speed of rotation of the input shaft compared to the speed of rotation of the output shaft.
It is known to use a transmission controller in an automatic transmission to monitor various vehicle operating conditions and driver inputs to determine whether a change in gear ratio, for example, an up-shift or a downshift, should occur. The transmission controller controls a plurality of clutches and brakes by selectively applying or releasing hydraulic pressure to effect changes in the gear ratio. During a downshift, an off-going component or clutch gradually disengages the present gear as an on-coming component or clutch gradually engages the next lower gear. Commonly, the transmission controller controls the disengagement or release of the off-going component based on the rate of engagement of the on-coming component. The downshift causes a numerical increase in the gear ratio, or an increase in the difference between the transmission input shaft speed compared to the transmission output shaft speed. For example, at a first speed in a third gear, the gear ratio may be 1.195:1, or if the input shaft speed is 3,000 rpm then the output shaft speed is about 2,510 rpm. Comparatively, in a second gear, the gear ratio may be 1.568:1. Therefore, at the first speed in the second gear, to maintain the output shaft speed of 2,510 rpm, the input shaft speed must rotate at about 3,936 rpm. The numerical increase in gear ratio causes the speed of the input shaft to rise in proportion to the ratio change.
It is known to arrange automatic transmissions to change gears, and thus speed ranges, dependent on various running conditions of the corresponding vehicle. Typically, a shift map is provided that is composed of up-shifting and downshifting lines for each speed range. The lines of the shift map may be established in relation to certain variables such as, for example, the vehicle speed, the opening of a throttle valve, and the intake vacuum, which may represent the engine-power output. Recently, automatic transmissions have been arranged to enable an operator to select both an automatic transmission mode wherein a gear change is automatically carried out and a manual transmission operation, or manual-shift, mode wherein gear-shifting is manually carried out by the vehicle operator. The manual-shift mode allows the operator to cause the automatic transmission to up-shift (referred to as a manual up-shift) or downshift (referred to as a manual downshift) independent of a predetermined shift map or other transmission control variables that the transmission controller may use to determine whether an up-shift or downshift should occur in the automatic transmission mode. The manual up or down shift may be controlled by manipulating a shift lever in a forward or backward direction or through similar type of manual shift control devices.
A manual downshift at a low accelerator pedal opening may occur when a vehicle operator manually initiates the gear ratio change, or downshift, for example, by manipulating the shift lever in the backward direction, while allowing the accelerator pedal to rise from the floor surface. The rising of the accelerator pedal allows the engine throttle opening to decrease in response to the movement of the pedal. The downshifting causes the transmission gear ratio to increase thereby causing the speed of the input shaft to increase. Because the engine is coupled to the automatic transmission via the input shaft, the engine speed is caused to increase as the speed of the input shaft increases. The energy required to accelerate the engine inertia and friction torque load is supplied by vehicle inertia in part due to the low accelerator pedal opening. This energy exchange causes the output torque supplied to the vehicle's ground engaging wheels to drop. The drop in output torque supplied to the vehicle's ground engaging wheels results in a bumpy undesirable feel commonly referred to as “shift-shock.”
What is needed then is a method for reducing the shift-shock associated with manual downshifting at low accelerator pedal openings.